I. Field of the Invention
This invention relates generally to freezers used in the manufacture and storage of food items. More particularly, this invention relates to such a freezing system for continuous automated freezing of food items, wherein cooling coils of the freezing system may be defrosted without interrupting the continuous automated freezing and without interrupting production of the food items.
II. Discussion of the Related Art
Blast freezing systems have been used in the past in food manufacturing operations. Such blast freezing systems require a constant temperature in the range of -30.degree. F. A refrigeration system of this type cannot maintain operation at this temperature without constant cooling from the cooling coils. At the low temperatures of this type of refrigeration system, any moisture within the system tends to build up and freeze on the cooling coils. When the layer of frost becomes too thick on the cooling coils, the coils no longer operate effectively to cool the system. Hence the cooling coils must periodically be defrosted.
With the prior art, blast freezing system's refrigeration coils may be required to be defrosted as often as several times a day. Defrosting the coils can take from one to four hours. With many such systems, manufacture of food products must cease during these defrost cycles because there is no way to immediately freeze the food product for storage.
One approach previously adopted to improve plant efficiency was to reduce the length of the defrost cycle of the blast freezing system. A common method of reducing the time required to defrost the cooling coils is the hot gas method. When the hot gas method is used, warm refrigerating gas, instead of the cold liquid, is pumped through the coils to warm the coils. The hot gas melts the ice and frost off the coils. One such hot gas defrost refrigeration system is disclosed by Lammert in U.S. Pat. No. 5,056,327. Another hot gas defrost system for refrigeration systems is disclosed by Gregory in U.S. Pat. No. 5,157,935.
The hot gas method reduces the duration of the defrost cycle. However, production must still cease during the defrost cycle. Therefore, it is desirable to have a freezing system which can be effectively defrosted and still permit food production to continue 24 hours a day. This can best be achieved by a system, such as the one contemplated by the present invention, which includes a plurality of cooling units or freezing systems constantly providing cold air to the system. The cooling units are sequentially defrosted so that, as one cooling unit is defrosted, the other cooling units continue refrigeration. The successful design of such a system requires that the defrosting coil of the associated cooling unit be isolated from the other cooling units and from the cold air produced by the other cooling units. Also, the chilled air generated by the cooling units remaining on-line must be sufficient to quickly freeze the food (i.e., -20.degree. F. to -30.degree. F.).
Louver arrangements used to isolate the defrosting coils require undue, expensive, and time consuming maintenance. Any such louver arrangement would require external power, additional programming, additional wiring, and frequent maintenance of moving parts. More importantly, moisture and frost would tend to collect on the louver arrangement constricting movement of the louvers and, thereby requiring additional defrost mechanisms on the louvers, to successfully use the louvers to isolate the coils as they are defrosted.
The present invention overcomes these disadvantages by providing a sequential defrost system and apparatus that may operate 24 hours a day, wherein the cooling units are isolated, without the use of louvers, thereby eliminating the requirements of external power, additional programming, additional wiring, or maintenance of moving parts.